1. Field of the Invention
The present invention relates to fibre optic couplers of the type comprising two or more optic fibres with their claddings fused together at an intersection of the fibres so that light injected into one fibre is coupled into the other fibre at the intersection, and to optical devices utilising such couplers.
2. Description of Related Art
The splitting ratio is largely determined by the degree of fusion of the fibre claddings, the length of the fused portion at the intersection, the numerical aperture of the fibres and the separation between the fibre cores at the intersection. Such couplers can be made by the fused biconical-taper technique, which involves laying one lightly tensioned optic fibre over another lightly tensioned optic fibre so that the fibres are nearly parallel at their intersection, and heating the fibres at the intersection to partially fuse their claddings together whilst pulling the fibre ends to elongate the resulting fused coupling region at the intersection. The elongation of the coupling region may be adjusted to achieve a desired splitting ratio. Such a method is described in detail in published UK Patent Application No. 2,136,985A for example. In a variant of this technique, one or more optic fibres may be twisted around each other and the twisted region heated to fuse the claddings whilst the fibres are tensioned to form the waist in the fused region. Such a method is disclosed in published UK Patent Application No. 2,112,165A. The above published UK patent applications are hereby incorporated by reference.
Fibre-optic couplers have been widely used in telecommunications, and because it is obviously undesirable that the splitting ratio in such couplers should be affected by the environment, fibre-optic couplers for use in telecommunications systems generally incorporate claddings which are almost completely fused together at the fibre intersection and are usually potted in a suitable potting compound. We have found that fibre optic couplers which have hitherto been used as passive optical components with a fixed splitting ratio, can be used as active components in optical transducers.